The present invention is directed to an arrangement for inductive guidance of non-track-bound vehicles in which a guiding conductor laid alongside a vehicle supporting surface is energized by an alternating current to generate a guiding magnetic field.
A recurrent problem with such systems is that the course of the guidance-determining alternating magnetic field is distorted in an uncontrolled fashion by concrete reinforcing steel which is integrated into the supporting surface, and the vehicle is guided in accordance with this "rolling course", which (at least at speeds above approximately 30 km/h) becomes increasingly troublesome with increasing speed, and is intolerable in the region of 80 km/h. German patent document No. DE 24 45 001 C2 discloses an arrangement for dealing with this problem in which two aerial crossed coils are provided vertically one above the other at a distance on the vehicle, and the actual position of the vehicle is determined in relation to the alternating magnetic field in each case independently using the two aerial signals. If the two events have different results, the difference can only be due to a locally interference-determined asymmetry of the alternating magnetic field. The actual transverse position of the vehicle in relation to the guidance-determining conductor can also be concluded from the difference between the results. When faults of the alternating magnetic field are detected, the actual position of the vehicle determined in this manner is further processed by an alternate method for the guidance of the vehicle.
A disadvantage of this arrangement is that during the evaluation of the two different aerial signals the differential value of two intermediate results is further processed, both intermediate results each being subject to a specific degree of measuring uncertainty. Although the absolute value of the difference between the two is extremely small, on the other hand the uncertainties may be added, so that the ratio between the reliable useful amount and the unreliable portion of the measuring result is extremely small. In addition, the relatively high vehicle-side expenditure can become very considerable in the case of a relatively large pool of guidable vehicles. Also, in some existing systems in which the problem of magnetic field interference has not occurred hitherto, (for example, because of the use of asphalt supporting surfaces or because concrete reinforcing steel lies deep below the surface of concrete supporting surfaces), the problem can arise subsequently because of extensions to the route network necessitating a retrofit of all the vehicles of the pool, which is expensive.
German patent document DE 25 00 792 C2 discloses a multi-conductor system for driverless transport systems, in which vehicles are guided inductively along an alternating magnetic field on the side of the supporting surface. Transport systems of this kind are used, for example, in production lines for the automation of the material flow; their vehicles travel at most at a walking pace, (that is to say very slowly in comparison with vehicles for roadworthy passenger or goods traffic). Magnetic field distortions at such low travel speeds do not cause any interference and the problem of balancing such magnetic field distortions does not arise.
In the previously known multi-conductor system, the individual conductors are arranged at a very small distance apart and can be represented, for example, by a two-conductor flat-ribbon radio aerial cable. The conductors of the conductor pair may be arranged either horizontally (one next to the other) or else vertically (one above the other). The known multiple arrangement of floor conductors is provided in order to guide the vehicles safely through route branches and junctions, without costly special arrangements of additional conductors in these regions and without having to connect or disconnect relatively long, impedance-varying branches of guiding conductors. The two conductors do not extend parallel to one another in the region of the branching, but divide in accordance with the desired course over the dividing routes; subsequently in each case one individual conductor which comes from elsewhere "joints" with the conductor which leads on individually to form a new conductor pair which then follows the desired course of the new route. The conductors are arranged in a network of routes provided with route branches and junctions, in such a way that crossing over of the conductors does not occur, and a single continuous conductor passes through all the routes in the entire network twice--even if on different paths.
There are no crossed coils arranged as aerials on the associated previously known vehicles. Rather, three vertically aligned aerial coils located one next to the other. Of these, only two adjacent coils are active at the same time: that is, the central coil and the right-hand one, or the central coil and the left-hand one. The two active coils together essentially sense the alternating magnetic field of only a single conductor of the conductor pair. Therefore, in each case only one of the two conductors or only one of the two magnetic fields is effective as a guiding field. Only in the case of a desired route change at a branch does the system change to the other conductor, by switching over to another pair of aerial coils, and from this time onwards this other conductor and the alternating magnetic field surrounding are used to guide the vehicle. Because of the distance between the supporting surface conductors is small the path of the conductors is not continuously parallel, and due to the different design and arrangement of the vehicle-side aerial coils, no improvement in any magnetic field distortions in the previously known system arising from interference is to be expected, especially since magnetic field distortions of this kind are in any case not a problem.
German patent document DE 25 02 405 A1 describes another inductive vehicle guiding system for non-track-bound vehicles, in particular tractors, which are provided with steerable wheels. Specifically, this publication addresses the problem of providing automatic guidance when cultivating fields, with a capability for easily changing the position of the working set course by a desired amount. For this purpose, at least two parallel guide cables, with an alternating current flowing through them always in the same direction, are laid in or under the surface plane (that is, the field to be worked), preferably along the field on both sides. The transverse spacing between the two guide cables is always an order of magnitude larger than the width of the vehicle. The guide cables are fed by a generator at one end and grounded at the other end. (Alternatively, a common return line can also be used.) Arranged on the vehicle side are aerial coils and an amplifier connected downstream, which permit the measurement of the magnitude and individual components of the magnetic fields originating from the guide cables. The height of the aerial coils above the supporting surface is negligible compared with the very large transverse distance between the guide cables, so that the strength of the magnetic field is measured virtually in the same plane as the guide cables; that is between the guide cables. In this arrangement, the aerial coils occupy a different position relative to the guide cables, namely to the left of one guide cable and to the right of the other guide cable, and opposite vertical components of the magnetic fields are detected along a set course. By means of an appropriate course correction which is induced in the vehicle, the opposite measurement signals are nulled, and the vehicle is thus kept on the set course. By preselection of a ratio of the current strength or of the frequency in the two guide cables, the position of the set course in relation to the guide cables can be varied infinitely. By means of the vehicle-side aerial coils, the geometric location of the vehicle between the guide cables (at which the strength of the two interfering magnetic fields is in each case of identical size) is continuously detected.
According to the foregoing patent document, both the current strength and the frequency can be used to change the magnetic field strength around the guide cables. This publication does not, however, address the problem of magnetic field interference, because this problem does not arise at all when cultivating fields: on the one hand in a field, a meadow or the like which is to be cultivated, there are normally no elements which cause magnetic interference and, on the other hand, any such elements, for example, a forgotten hoe or the like, would interfere with the vehicle course only to a degree which is in any case tolerable. A lateral offset of a few centimeters due to interference would adversely affect neither the working result of the cultivation of the field nor, because of the low working speed, the driving comfort on the tractor.
German patent document DE 26 08 008 A1 describes an inductive vehicle guiding system for boats for leisure parks. In this arrangement, the guidance conductors are laid in a plane underneath the vehicles, for example at the same depth under water. Depending on the number of passengers, the boats are submerged in the water to a greater or lesser depth, and thus, the problem arises that the vertical distance between the aerial coils and the conductors (and hence the strength of the aerial signals) varies. Also, a greater or lesser degree of pollution of the water, in particular by metallic foils, influences the strength of the aerial signals.
In order to compensate for such interference factors which affect signal amplitudes, a total of four guidance conductors are laid in a plane underneath the path of the boat. In each case two conductors are connected at the end of the route to form a conductor loop, and alternating current thus flows through them in opposite directions. The four conductors are therefore laid in pairs in conductor loops which themselves are fed with alternating currents that are offset by 90.degree.. The four conductors therefore have alternating currents flowing through them which have the same frequency, but differ in phase angle by 90.degree., specifically with the phases 0.degree., 90.degree., 180.degree. and 270.degree.. This patent does not give any more detailed information on the transverse position of the individual conductors in relation to the set course of the vehicle; all that can be assumed on the basis of the drawing and the overall context is that the four conductors are to be laid symmetrically with respect to the set course.
In this system, means of the aerial coils which are installed on the boat, it is not the amplitudes of the magnetic field strengths but rather the phase angle which is evaluated for purposes of guiding the boat. Depending on the size and direction of the transverse deviation of the aerial coils from the set position, phase signals of the magnetic field vectors are produced which vary in size and direction. What is important in this case is simply the magnitude of a useful signal, but not the actual representation of the signal for a transverse position error of the vehicle. Since such interference sources are not present, due to the absence of interfering iron particles in the vicinity of the guidance-determining conductors, the problem of magnetic field interference, and of a corresponding interference in the guidance during automatic steering of the boat, does not arise. Moreover, any such interference would have a complete superimposition of normal pitching and following movements of the boat caused by waves; and therefore would not be perceptible at all.
The object of the present invention is to provide an arrangement for inductive guidance of roadworthy non-track-bound vehicles which compensates for local supporting surface interference of the guidance-determining alternating magnetic field, without need of complex vehicle-side measures, particularly multiple arrangements of aerial crossed coils and special evaluation channels for each of the aerial signals for the equalization or compensation of the influence of the magnetic field interference.
This object is achieved according to the invention in an arrangement of the generic type described heretofore for inductive guidance of non-track-bound vehicles, by an arrangement of the guidance conductors which minimizes local interference generated by reinforcement elements in the supporting surface without requiring additional circuits or detection coils. With a multiple arrangement of conductors, the alternating magnetic field decreases more rapidly as the distance from the conductor increases than is the case for a single-conductor system; but the interference of the magnetic field in the case of the multi-conductor system also decreases correspondingly more rapidly than is the case in the single-conductor system. In fact, in the case of a two conductor system interference decreases with the square of the distance of the conductor from the interfering element, while in the case of the three-conductor system it decreases with the cube of the distance, and in the case of the four-conductor system the power of four etc. The alternating magnetic field component (that is, the horizontal or vertical component, depending on the arrangement of the conductors), which is used for guidance, is approximately the same size in the case of a multiple arrangement of conductors under comparable conditions as in the case of a single-conductor system. Thus, interference generated in the supporting surface affects the alternating magnetic field considerably less in the radial direction in the multi-conductor system than is the case in the single-conductor system. In the case of the multi-conductor system, the interference is more localized in an area near to the conductor than is the case in the single-conductor system; and hence, despite any supporting surface generated magnetic field interference, virtually an undisturbed alternating magnetic field with an uninterrupted path is present at the vehicle aerial crossed coils.
The preferably two conductors should have a certain minimum spacing, but must however not be too far away from one another. If the mutual spacing of the conductors is too small, differences in level between the conductors have perceptible influences even on the transverse position of the alternating magnetic field; in addition, the useful field strength drops under otherwise comparable conditions with diminishing transverse spacing of the conductors. If the transverse spacing of the conductors is too great (for example significantly greater than the vertical spacing between the aerial crossed coils and the supporting surface), the slope of the characteristic line for transverse deviations of the vehicle from its desired position on the supporting surface is too small. With the stated relative transverse spacing of the conductors of approximately 50 to 100% of the height of the aerial crossed coils above the supporting surface, both the useful field strength and the ratio of interference field strength to the slope of the characteristic line are near to the theoretical, but virtually unattainable, optimum values.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.